Unit 23: Critical Distance

Critical Distance

Critical distance is the distance from where the direct and resonant sound can becomes one.

The sound from the speaker and reflections of the room sound equal. The critical distance varies from room to room. If the room is acoustically treated well and can absorb more sound then the critical distance will be further whereas if its not then the distance will be shorter

For this to be effective the critical distance should be far from the sound source as possible so that less reverb is produced. Sound from the speaker dies down over distance whereas reverb spreads through the room remaining constant. As a result of new sound coming from the speakers the reverb builds up, right until the sound produced equal the sound being absorbed.

When reverberant sound reaches 12db or exceed the direct sound clarity is lost. The sound produced is no longer clear and much harder to understand, so finding the critical distance before shows is vital. You can find it by playing 'compressed pop music' through the speakers. Start of with either one then walk around and identify it. Then repeat this step with the other speaker, them both.

Critical Distance:   Dc = 0.14/√QR   (Q = directivity factor 1 of sound source.   R = room constant)



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